The Collaborative International Dictionary of English v.0.48:

Parasitic \Par`a*sit"ic\ (p[a^]r`[.a]*s[i^]t"[i^]k), Parasitical
\Par`a*sit"ic*al\ (p[a^]r`[.a]*s[i^]t"[i^]*kal), a. [L.
   parasiticus, Gr. ?: cf. F. parasitique.]
   [1913 Webster]
   1. Of the nature of a parasite; having the habits of a
      parasite; fawning for food or favors; sycophantic.
      "Parasitic preachers." --Milton.

   Syn: leechlike, bloodsucking.
        [1913 Webster]

   2. (Bot. & Zool.) Of or pertaining to parasites; living on,
      or deriving nourishment from, some other living animal or
      plant. See Parasite, 2 & 3.
      [1913 Webster]

   Parasitic gull, Parasitic jager. (Zool.) See Jager.
      [1913 Webster] -- Par`a*sit"ic*al*ly, adv. --
      Par`a*sit"ic*al*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

parasitical
    adj 1: relating to or caused by parasites; "parasitic infection"
           [syn: parasitic, parasitical]
    2: of plants or persons; having the nature or habits of a
       parasite or leech; living off another; "a wealthy class
       parasitic upon the labor of the masses"; "parasitic vines
       that strangle the trees"; "bloodsucking blackmailer"; "his
       indolent leechlike existence" [syn: parasitic,
       parasitical, leechlike, bloodsucking]